Because the atmosphere of the Earth often carries dust, and otherwise it acts just the way it does when you try to look towards the west near sunset. The blue component of the light from the Sun is scattered out of the light you receive giving it an orange-red-crimson hue. When sunlight passes through the Earth's atmosphere on its way to the Moon during an eclipse, the blue component has been refracted out by atmospheric dust and aerosols, giving the Moon a pinkish or red hue.